1. Field of the Invention
This invention relates generally to electric motors. More specifically, the invention relates to a design for reducing noise production in running electric motors.
2. Description of Related Art
One of the most daunting problems faced by motor designers concerns the production of noise and associated vibration by various motor components. One of the many sources of noise/vibration is derived from the axial movement of an electric motor's rotor/shaft assembly during motor operation within the boundaries of the bearings or other shaft supporting mechanisms employed. The play that has to be designed into a motor in order to allow for rotor/shaft rotation inevitably provides the environment for motor shaft axial movement.
The axial movement of an electric motor's rotor/shaft assembly also leads to considerable wear of the components that are contacted by the rotating shaft. The components include bearings and items such as impellers that are attached to the shaft. Constant axial movement, apart from creating the possibility of more rapid wear of motor components also magnifies any undesirable noise generated by the components attached to the shaft that are designed to move radially about the shaft.
This problem, which is present in virtually all electric motors, is particularly pronounced in electric motors that are run on AC electric current. It is well known that a motor operating on a typical 60 Hz AC signal develops considerable noise at twice line frequency. FIGS. 1 and 2 are graphs which illustrate typical sound signatures generated by common shaded pole motors. As is clearly shown, at twice line frequency, the decibel levels achieved by the motors reach approximately 70 and 76 decibels, respectively. These are common but unacceptable decibel levels.
Numerous techniques have been employed to address this problem of motor design. One way of minimizing the problem is to adhere or mechanically affix roller or sleeve bearings to the shaft and set the bearings in bearing seats having very tight tolerances. An example of a typical "sleeve bearing" is shown in FIG. 4 where a shaft 3 is secured by sleeve bearings 4. An example of a typical "ball bearing" is shown in FIG. 5 where the shaft 3 is secured by ball bearings 5. This approach attempts to "lock" the shaft in place by virtue of very tight tolerances. Unfortunately, it only takes one two-thousandth of an inch play to enable a running motor to create noise via axial movement of the rotor/shaft assembly. Although this method can reduce some of the undesired noise production, it also creates the possibility of greater wear of the rotating components and the components within which the rotating components rotate.
Another technique used to reduce noise, albeit not at twice line frequency, is described in U.S. Pat. No. 5,497,039 to Blaettner et al. In the '039 patent, the armature/shaft assembly of a DC motor is preloaded to force the shaft against one end of the motor. This is accomplished by employing an armature/permanent magnet arrangement whereby the permanent magnets affixed to the casing of the DC motor overhang the armature in the axial direction. The armature is then offset to the magnets to create the preload condition which causes the armature to be forced into one end of the motor casing. To compensate or, more accurately, to dampen the effect of the preload force on the motor casing end against which the armature is being forced, a combination of an elastomeric washer and a hard plastic washer is used.
Although this method will eliminate some of the axial noise/vibration, it ultimately does not eliminate an acceptable amount of noise/vibration as is graphically illustrated in FIGS. 6-9. It does not address the problem of noise at twice line frequency. A more detailed explanation of the loss is provided below.
The described methods for reducing noise/vibration production via oscillation/axial movement of the rotor/shaft assembly are all marginally beneficial but with discernable drawbacks. A design and method has now been discovered which significantly reduces the noise/vibration production while minimizing the detrimental effects exhibited by the other known designs and methods.
It is thus an object of the invention to provide a motor design and method for reducing the noise/vibration that is generated by the axial movement of a rotating shaft within an electric motor.
It is a further object of the invention to significantly reduce the noise/vibration generated by an AC motor at twice line frequency.
It is another object of the invention to provide a design which enhances manufacturability and cost-effectiveness of an electric motor.